In the manufacture of VLSI devices, refractory metal silicides have been used as interconnection materials to overcome disadvantages of a polycrystalline silicon (hereinafter referred to as polysilicon). Since the polysilicon has a sheet resistance of 20 to 30 .OMEGA./.quadrature. for a thickness of 5000 .ANG., it is very difficulty to achieve the reduction of R-C delay time for a high speed operation and the scaling-down of line widths for higher density. Therefore, metal silicides such as tunsten silicide, titanium silicide, platnium silicide and tantalum silicide, which may provide a lower sheet resistance, by one order of magnitude, than that of the polysilicon, have been employed to achieve the scaling-down and the high speed operation in on-chip VLSI devices.
However, some problems for metal silicides should be taken into consideration to accomplish stable and safe ohmic contacts through a silicon dioxde layer between the metal silicide and heavily doped preohmic regions in a silicon substrate.
Firstly, in order follow a CMOS fabrication process which is most widely used in the manufacture of VLSI devices, a metal silicide capable of simultaneously making ohmic contacts with N+ and P+ preohmic regions should be selected. In the prior art, the tungsten silicide has been used as a contract material for N+ preohmic regions. However, since the tungsten silicide out-diffuses dopants from N+ and P+ preohmic regions during a silicidation process requiring high temperature treatment, the contact resistance between the tungsten silicide and the preohmic regions increases.
Secondly, sputter-deposited silicides do not always provide a good step coverage on vertical side-walls of the silicon dioxide, while metal silicides formed by chemical vapor deposition (CVD) generally have good step coverage thereon.
It has been found that the titanium silicide of metal silicides has the lowest sheet resistance. Two methods of forming the titanium silicide by the sputtering technology have been known in the prior art. One is to thermally react sputter-deposited titanium with the underlying silicon. The other is to directly deposit the titanium silicide by sputtering. However, in any case, the sputter-deposited titanium may not provide a good step coverage on vertical side-walls of the silicon dioxide of about 5000 .ANG. in thickness and may cause a serious result of electrical disconnection.